Soft x-ray tube



Jan. 2, 1934. c. M. SLACK SOFT X-RAY TUBE Filed Jan. 9, 1930 2 Sheets-Sheet l I l I INVENTOR CIVLSLHck ATTORN Y 1934- c. M. SLACK 1,942,007

SOFT X-RAY TUBE Filed Jan. 9, 1930 2 Sheets-Sheet 2 INVENTOR C./VI.$LACK ATTORNE Patented Jan. 2,

SOFT X-BAY TUBE 7 Charles M. Slack, Bloomfield, N. .L, 'assignor to 1 Westinghouse Lamp Company, a corporation of Pennsylvania Application January 9, 1930. Serial No. 419,479

- A 1 Claim.

The present invention relates to devices employed in the production of X-rays of long wave length, usually termed soft X-rays or Grenz rays, and particularly to the production of X- 5 rays of from 2 to 8 angstroms in wave length.

For certain medical purposes it is desirable to produce soft X-rays since these have a low penc trating power. Hard X-ray tubes are not suitable for these purposes since the short wave length rays penetrate too deeply and very few are absorbed by superficial parts of the body treated.

Within an X-ray tube of the hot cathode, high vacuum type the hardness of the rays produced depends on the impressed voltage. By impress- "ing less than ten kilovolts upon the tube only the soft X-rays will be produced therein. The glass walls of conventional X-ray tubes, however, are so thick that they absorb practically allof ....the soft X-rays produced. Soft X-ray tubes have been provided with windows of various materials, such as aluminium foil and Lindemann glass for the projection of soft X-rays into the open air. Wehnelt and ,iTrenkle in publication entitled The Production of Very Soft Roentgen Rays appearing in the Erlanger Physical Institute of the University November, 1905, described a vacuum tube of the hot cathode type having an aluminium foil win- ,.dow attached thereto, which tube was capable of projecting soft X-rays through thewindow when the tube was operated with an impressed potential difference of about 7,000 volts. Lindemann glass windows for soft. X-ray tubes have -been proposed in Bucky Patents 1,679,332, 1,679,-

894 and 1,708,494 issued July 31,1928, August 7,

1928 and April 9, 1929.

Aluminium foil of a thinness of 0.004 m. m. or about 0.00015 inches such as employed by Weh- ...nelt and Trenkle is extremely fragile and if of 40 any appreciable area, it must be supported by a.

heavier metal structure to which it must be secured in a gas tight manner. The metallic structure, in turn, must also be sealed to the glass .portion of the envelope in a gas tight manner. This construction is both costly and difficult to effect. Difficulty is also experienced in degasifying the metal parts within the tube without destroying the window. Lindemann glass windows present the disadvantage that due to the brittleness of this material a window capable of withstanding atmospheric pressure is limited in thickness to a minimum of about 0.008 inches at which thickness a major portion of the rays of 3 angstrom wave length are absorbed by the window. From a manufacturing standpoint Lindemann glass presents a further disadvantage in that it is difficult to effect a good seal between a thin film of this material and the ordinary glass portions of a tube.

Another disadvantage of Lindemann glass is that it deteriorates on exposure to atmosphere and must be coated with some such material as shellac or lacquer to protect it This coating must be renewed frequently or the rays themselves disintegrate the coating material.

One of the objectsof my invention is to provide an improved window for a soft X-ray tube which will project a major portion of the X-rays produced through the same.

Another object of my invention is to overcome the difficulties inherent in the windows in use at the present time and to employ a portion of the glass envelope of the device through which soft X-rays may be projected.

A still further object is to produce a window 7 for a soft X-ray tube which may be made extremely thin while retaining suflicient strength to resist the external atmospheric pressure and which will absorb but a very small proportion of the soft X-raysproduced in the tube.

According to my invention I provide a window permeable to soft X-rays and composed of glass or other vitreous material of extreme thinness such as the one described in my Patent 1" 1,735,302 issued November 12, 1929. 5

Briefly this window consists in an indrawn bulbous portionon the end of the tube of a thickness of from 0.0001 to .005 inches.

For the purposes of the present invention the thickness should be less than 2 mils since with a wave length of 2 angstroms, the most penetrating of those considered, a window 10 mils thick will transmit less than one part in a thousand of the radiations, a window 5 mils thick will transmit less than one percent, a 2 mil window will transmit 14 percent, a 1 mil window 40 percent, and a 0.1 mil window will transmit'about 90 percent.

At a wave length of 3 angstroms 1 mil of glass will transmit about 5 percent of the radiations, 0.1 mil about 80 percent and 0.04 mils about 90 percent'of the radiations.

In order that the invention may be more fully understood, reference will be had to the accompanying drawings in which;

Fig. 1 is a central sectional view of a soft X- ray tube constructed in accordance with my invention, and

Figs. 2 and 3 are similar views on an enlarged scale disclosing other embodiments of my invention.

- The tube shown in Fig. 1 comprises a glass envelope 1 having a relatively heavy wall and containing a filamentary cathode 2 and an anode 3. The anode supporting end 4 of the tube has a reentrant glass stem 5 terminating in a press 6 through which a leading in conductor 7 for the anode is sealed.

The envelope 1 may be composed of any suit able glass, such as ordinary lime glass, pyrex or a glass which is less opaque to X-rays.

The cathode 2 may be of any suitable electron emitting material, but I prefer to construct it of tungsten or tantalum in the form of coil 8 mounted within a focusing cup 9 as is usual in X-ray tube construction. The focusing cup 9 is supported by leading-in and supporting oonduo: tors 10 which are sealed in the press 12 of an outwardly projecting stem 11 located at the side of the tube adjacent the end 13 which is opposite the anode supporting end 4.

The end 13 of the tube may be e lar ed and the Window 14 formed therein. In forming window 14 the enlarged end is heated by a blow pipe flame or other suitable heating means and a suction created on the interior thereof so that the plastic glass is drawn in, the heating and suction being continued until the requisite size and thickness of the bulbous portion of the tube is obtained. The Window 14 may preferably have a thickness of from 0.000104 to 0.002 inches. By properly controlling the application of heat a considerable area of the central portion of the A substantially uniform thickness, the walls gradually increasing in thickness from the central portion to the outer edge of the bulbous portions.

The leading in and supporting conductors l0- may be ofiset as shown at 15 so as to dispose the. cathode 2 in close. proximity to "the. window 14 and at the same time. incline the focusing cup 9 away from the window so that cathode rays emerging from the filament 8 cannot strike the window, the filament being withdrawn into the focusing cup 9 to, eflfiect this result. The foc u sing cup 9 directs cathode rays to. the centerof a target 16; of tungsten or thorium or other suitable material secured to the anode 3 thru efi rad at r 1 Water. 0.0 m devic may. h added in mann r wel ne rstqod inthe art.

he pur o of sp eadin the over,-

a large area the window '14, is made shall w.- and i de mou h.- e. elec rqd s a also d s: posed closely adjacent the windowso. that the e ay mitted b he. tube orm. a l rg solid angle.

Referring to Fig. 2 of the drawings there is illustrated a soft X-ray tube embodying my invention and particularly designated for treatment of the mouth and throat. This embodiment comprises a glass envelope 20 ofcylindrical shape and containsa cylindricar glass tube 21 supported at one end. in the envelope 20 by a sleeve 22. o 75 The free end of tube 21 supports on itsexwindow may be drawn very thin and have a.

the intermediary. of a bonding d-is'c 137 which may- Leading-in conductors 28 for the coil 27 pass through apertures 29 in the end wall of shield 23 and emerge from the envelope 20 at about the central'portion thereof to join suitable terminals 30. An anode 31is centrally supported within the tube 21 by a sleeve 32 and a leading-in conductor 33 for the anode 31 is sealed in the press 35 of a, reentrant stem 34 in the same manner as previously described relative to elements 5, 6, '7 of Fig. 1.

A thin window 36 similar 1 is formed in the end of the envelope 20 opposite the anode supporting end and in alignment with the aperture 24, a filament coil 27 and anode 31. The terminals 3Q are disposed remote from; said window to permit insertion of the window 36 and the portion of the envelope supporting the window into the mouth.

In Fig. 3 another embodiment of my invention designed particularly for treatment of small areas such as those within the nose or ear is illustrated. This tube may be constructed in essentially the same manner as the tube shown in Fig. 2 except for the window structure which comprises a'sniall thin window 36" formed in a 1 10 tube 3'7 of about a half inch or less in diameter. Preferably the window 36 isformed on a piece of glass tube 37 and the tube, subsequently fused; to an envelope 20-.

In the above described devices the electrodes should be thoroughly degasilied in accordance with the practice followed in the construction 01' X--ray tubes. Prior to assembly the metal parts are heated to a red heat to remove the major portion of the occluded gases and. after assem- 129 bly the electrodes and associated parts are sealed into the envelope, a high vacuum created and the final degasification efifected' by electronic bombardment and high frequency induction heat-.

ing while the tube is on the exhaust pumps.125 After the final degasification the tube may be sealed off from the pumps and constant. operating conditions ma be obtained when the tube isworkedat the potential necessary for the production of soft X-rays. If desired, a small pres,-. sure of from 1 to 10 microns of a monatimic gas. such as neon may be admitted to the envelope. In cit-her case the arrangement of the electrodes and gas pressures employed are such, that the device may be operated to produce X-rays in the absence of detrimental gaseous ionization. T

In operation when a 'smallpotential difference is impressed on the cathode leading-in wires anda desired potential difference impressed on the anode and cathodoleading in conductors X-rays of'- a known minimum wave length w l be p oduced within the tube. An extremely thin window will absorbbut a; very few of the longer, wave length x ra-ys so that the majorportionofthe soft rays areavailable for treatment purposes. The thicker the window is the greater the numberofthe softer rays which will be ab-, sorbed by the window andthe smaller the emi- V ciency of the tube. Asthepotentialdiiferences impressed on the anode and; cathode are in- 159 95 to window 14 of Fig.

creased the minimum wave lengths of the radiations produced are decreased and a greater proportion of the hard penetrating rays are produced.

If, for example, radiations in the region of 2 to 4 angstroms wave length are desired a potential difierence of 6 kilovolts may be impressed on the anode and cathode and by using a window of 0.00004 inches thickness about ninety percent of these radiations produced will pass through the window. If radiations of a minimum wave length of 8 angstroms are desired about 1.5 kilovolts potential difference is impressed on the electrodes. With the same window a lower percentage of the rays will be emitted by the tube. I have found that the windows having silicon as the predominating element ofier the greatest resistance to the passage of X-rays in the neighborhood of 7 angstroms wave length, and at about that point the absorption coefficient drops so that for 8, 9, 10, 11 and 12 angstroms more of the rays will pass through the window than will at 7 angstroms.

It is to be understood that although my tube is particularly designed for the production of X-rays of from 2 to 8 angstroms, it is capable of transmitting radiations of a greater wave length, such as those from 8 to 15 angstroms.

What I claim is:

An X-ray tube comprising an envelope of cylindrical form, a cathode and an anode within said envelope, leading-in conductors for said anode and cathode, the leading-in conductor for said anode extending through one end of the envelope and the leading-in conductors for said cathode extending from the sides of the envelope at diametrically opposed points, said envelope comprising a body portion containing an element whose atomic weight is greater than 16 and a window at the end of said envelope and of substantially the same composition as said body portion, said window being homogeneously united to said body portion and having a portion thereof of not greater than 0.002 inches in thickness, said leading-in conductors emerging from the envelope at points remote from the window.

CHARLES M. SLACK. 

